The treaty between Canada and the United States comes into effect on April 6, 1851, at a rate of 6 pence or 10 cents. US stamps remain available at Canadian post offices.
On April 23 the first stamp of the Province of Canada is issued. It is worth 3 cents which is the basic rate for domestic postage. With its vignette displaying a beaver, it is the first official stamp in the world to be illustrated on a theme other than portraits of personalities and national emblems.
Reverend Ryerson, a prominent Canadian Methodist educator, is temporarily in London. Somewhere in Canada, someone manages to send him a letter. The new rules must be applied but it is a little complicated. The letter will have to be embarked in New York for crossing the Atlantic.
The local postmaster accepts the postage in the following conditions. A 3-cent beaver stamp pays the transfer to the United States border. The letter will only transit through the United States and the journey between border and New York is not considered. The departure of a British steamer of the Cunard company is scheduled for May 7 and this information is written on the envelope.
The Transatlantic rate of 24 cents is paid by five 5 cent stamps. On March 3 an act of the US Congress had announced the imminent obsolescence of this original denomination of the US Postage and consequently the sender chose to pay 1 cent more than required by the US-British postal treaty of 1848, certainly for using his stock.
The transfer to New York is fast. On May 6 the letter is embarked on a US ship. Despite the 1848 treaty, competition is fierce. The New York foreign mail office collects 22 cents in this operation, avoids paying 19 cents to the Cunard, offers a discount of 2 cents and hand stamps on the cover an overpayment of 3 cents. The receipt of the letter in London is recorded by a date hand stamp on May 19.
This piece surfaces in 1944 in an auction. Known as the Beaver Cover, it is a supreme delight in philately : very fine condition, paid with a mix of stamps from the very first US and Canadian federal issues, and bearing the marks and cancellations that reveal the significant details of its routing.
The Beaver Cover was sold for $ 715K including premium by Christie's Robson Lowe on September 28, 1993. It is estimated $ 600K for sale by Robert A. Siegel in New York on October 3, lot 26. Here is the link to the section dedicated to the Gross collection on the website of the auction house.